Go to device manager and check for yellow exclamation marks, if anything is there,
Install the latest audio device drivers for your mobo/system & check.
Go to control panel - select sounds and audio devices - Audio Tab- Check for Default audio. it must show something like Sigmatel Audio/Realtek Audio. etc,
For displaying sound icon in taskbar,
Go to control panel - select sounds and audio devices -check the Place volume icon in task bar.